Trichosanthin (TCS) is originally isolated from the root tuber of a Chinese medicinal herb Trichosanthes kirilowii Maximowicz and is identified as the active component of Tian Hua Fen, a Chinese medicine described and used clinically as early as two thousand years ago. Chemically, TCS is a 27 kDa sized type I ribosome-inactivating protein (RIP). It possesses RNA-N-glycosidase activity that inactivates the 605 subunits of the eukaryotic ribosomes (Zhang J S, Liu W Y, The Mechanism of Action Trichosanthin on Eukaryotic Ribosome RNA N-glycosidase of the Cytotoxin, Nucleic Acids Res, 20 (6): 1271–1275, 1992). Native TCS is composed of 247 amino acid residues. Its primary structure is shown in FIG. 1 (Nie H L, et al., The Cloning and Structural Analysis of Trichosanthin Gene, The 4th China Conference on Gene Structure Cloning and Expression, Haikou, A-23, 1991). TCS has been used clinically in China since the 1970's to induce mid-term abortion and to treat diseases of trophoblastic origin, e.g., hydatiform mole (Second Research Group of Shanghai Institute of Experimental Biology, Science in China, 19: 811–830, 1976). Soon after the laboratory finding in 1989 that TCS appeared to inhibit the HIV-1 replication in both acutely infected T-lymphoblastoid cells and in chronically infected macrophages (McGrath M S, Hwang K M, Caldwell S E, et al., GLQ233: An Inhibitor of Human Immunodeficiency Virus Replication in Acutely and Chronically Infected Cells of Lymphocyte and Mononuclear Phagocyte Lineage, Proc. Natl. Acad. Sci. USA, 86: 2844–2848, 1989), clinical trials of TCS as a potential treatment for AIDS were carried out. In addition to HIV, TCS is capable of attacking other types of virus. It was also found toxic to leukemia cells and other types of tumor cells (Kong M, Ke Y B, Zhou M Y, et al., Study on Trichosanthin Induced Apoptosis of Leukemia K562 Cells, Acta Biologiae Experimentalis Sinica, 31(3): 233–243, 1998; Zheng Y T, Zhang K L, Ben K L, et al., In Vitro Immunotoxicity and Cytotoxicity of Trichosanthin Against Human Normal Immunocytes and Leukemia-lymphoma Cells, Immunopharmacology and Inimunotoxicology, 17 (1): 69–79, 1995; Wu Y X, Xiang D N, Zhang S P, et al., The Toxic Effect and Its Mechanism of Trichosanthin Against Stomach and Colon Cancer Cells, Chinese Journal of Digestion, 13 (3): 263–266, 1993). In clinical uses, however, a dangerous complication was observed with the drug. It can occasionally cause immediate type allergic reaction mediated by innimnoglobulin E (IgE) antibody. The TCS specific IgE reacts to TCS in the body, initiating the onset of type I hypersensitivity manifested clinically as complications such as allergic urticaria, angioedema, and anaphylactic shock—a sudden, severe life-threatening allergic reaction that can kill within minutes. This dysfunctional immune response to TCS usually remains strong in the recipient's body for many years. As a result, TCS is restricted to only one administration during the recipient's lifetime as an abortifacient. Not only in abortion, allergic reactions were also present when TCS was used in treating other diseases. Its application was therefore greatly restricted.
The purpose of the present invention is to remove the side effects of TCS by developing a novel TCS product with reduced antigenicity that allows safe multiple administrations.